Buggy code:
public interface ServerAPI { @ FormUrlEncoded @ POST ( "//login" ) void login ( @ Field ( "email" ) String email , @ Field ( "password" ) String pw , Callback < Response_User > callback ) { } @ FormUrlEncoded @ POST ( "//category" ) void createCategory ( @ Header ( "Authorization" ) String authorization , @ Field ( "name" ) String category_name , Callback < Response_Category > callback ) { } @ GET ( "//category//" ) void getAllCategory ( @ Header ( "Authorization" ) String authorization , Callback < Response_CategoryList > callback ) { } @ GET ( "//category//{id}" ) void getCategory ( @ Header ( "Authorization" ) String authorization , @ Path ( "id" ) int id , Callback < Response_Category > callback ) { } @ FormUrlEncoded @ PUT ( "//category//{id}" ) void updateCategory ( @ Header ( "Authorization" ) String authorization , @ Path ( "id" ) int id , @ Field ( "name" ) String name , Callback < Response_Category > callback ) { } @ FormUrlEncoded @ POST ( "//inventory" ) void createInventory ( @ Header ( "Authorization" ) String authorization , @ Field ( "name" ) String name , @ Field ( "price" ) double price , @ Field ( "stock" ) int stock , @ Field ( "image" ) String image , @ Field ( "category" ) int category , Callback < Response_Inventory > callback ) { } @ GET ( "//inventory//{id}" ) void getInventory ( @ Header ( "Authorization" ) String authorization , @ Path ( "id" ) int id , Callback < Response_Inventory > callback ) { } @ GET ( "//inventory//" ) void getAllInventory ( @ Header ( "Authorization" ) String authorization , Callback < Response_InventoryList > callback ) { } @ FormUrlEncoded @ PUT ( "//inventory//{id}" ) void updateInventory ( @ Header ( "Authorization" ) String authorization , @ Path ( "id" ) int id , @ Field ( "name" ) String name , @ Field ( "price" ) double price , @ Field ( "stock" ) int stock , @ Field ( "status" ) int status , @ Field ( "category" ) int category , Callback < Response_Inventory > callback ) { } @ FormUrlEncoded <START_BUG> @ PUT ( "//inventory//{id}" ) <END_BUG> void updateInventoryImage ( @ Header ( "Authorization" ) String authorization , @ Path ( "id" ) int id , @ Field ( "image" ) String image , Callback < Response_Inventory > callback ) ; @ GET ( "//invoice//{id}" ) void getInvoice ( @ Header ( "Authorization" ) String authorization , @ Path ( "id" ) int id , Callback < Response_Invoice > callback ) { } @ GET ( "//invoice//" ) void getAllInvoice ( @ Header ( "Authorization" ) String authorization , Callback < Response_InvoiceList > callback ) { } @ GET ( "//invoice//month//" ) void getMonthlyInvoice ( @ Header ( "Authorization" ) String authorization , Callback < Response_InvoiceList > callback ) { } @ GET ( "//invoice//day//" ) void getDailyInvoice ( @ Header ( "Authorization" ) String authorization , Callback < Response_InvoiceList > callback ) { } @ FormUrlEncoded @ POST ( "//invoice" ) void createInvoice ( @ Header ( "Authorization" ) String authorization , @ Field ( "total_price" ) double total_price , @ Field ( "final_price" ) double final_price , @ Field ( "paid" ) double paid , @ Field ( "date_time" ) String date_time , @ Field ( "content" ) String content , @ Field ( "email" ) String email , Callback < Response_Invoice > callback ) { } @ FormUrlEncoded @ PUT ( "//invoice//{id}" ) void updateInvoice ( @ Header ( "Authorization" ) String authorization , @ Path ( "id" ) int id , @ Field ( "total_price" ) double total_price , @ Field ( "final_price" ) double final_price , @ Field ( "paid" ) double paid , @ Field ( "date_time" ) String date_time , @ Field ( "content" ) String content , @ Field ( "email" ) String email , @ Field ( "status" ) int status , Callback < Response_Invoice > callback ) { } @ Multipart @ POST ( "//upload//{id}" ) void uploadImage ( @ Header ( "Authorization" ) String authorization , @ Path ( "id" ) int inven_id , @ Part ( "file" ) TypedFile file , Callback < Response_FileUpload > callback ) { } @ GET ( "//uploads//{name}" ) void downloadAt_uploads ( @ Path ( "name" ) String fileName , Callback < Response > callback ) { } @ GET ( "//uploads//{name}" ) Response downloadAt_uploads ( @ Path ( "name" ) String fileName ) { } @ FormUrlEncoded @ POST ( "//gcm_register" ) void registerGCM ( @ Field ( "gcmid" ) String gcmid , Callback < Response_Message > callback ) { } @ FormUrlEncoded @ POST ( "//gcm" ) void sendGCM ( @ Header ( "Authorization" ) String authorization , @ Field ( "id" ) int inven_id , @ Field ( "statusCode" ) int statusCode , Callback < Response > callback ) { } @ FormUrlEncoded @ POST ( "//company" ) void updateCompany ( @ Field ( "name" ) String name , @ Field ( "email" ) String email , @ Field ( "phone" ) String phone , @ Field ( "address" ) String address , Callback < Response_Company > callback ) { } @ GET ( "//company//" ) void getCompany ( Callback < Response_Company > callback ) { } }

Fix:
@ PUT ( "//inventoryImage//{id}" )